Re: [Xen-devel] [PATCH] Fix mem.c so that X Windows can restart

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 25 May 2006, at 20:32, Donald D. Dugger wrote:

> This patch fixes the problem where you cannot start X Windows on Dom0 
> after
> you have created an HVM guest.  The problem is that X uses `mmap' to 
> map
> 1 page at physical address 0 with read/write permission.  Before an HVM
> guest is created this check at around line 1496 of `mm.c':
>
>         if ( unlikely((x & (PGT_type_mask|PGT_va_mask)) != type) )
>
> causes the offending code from this patch to be bypassed and X's `mmap'
> call works.  After an HVM guest is created the check at 1496 of `mm.c' 
> is
> now true and the code from this patch is executed, causing the `mmap' 
> call to
> erroneously fail.
This is totally bogus. That test (line 1496) should *never* be true for 
pages that are not type PGT_l*_page_table. I think some HVM code is 
indexing frame_table with a zero frame number and then fiddling with 
fields in that very first page_info structure. That's very bad -- in 
fact it's tempting to add some assertions about page 0 to be tested at 
various suitable moments to prevent this sort of bad behaviour.
 -- Keir


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